quick help for weird problem.

From: Jeffrey Barrett (one2beatyou_at_hotmail.com)
Date: 07/31/04 

Date: 30 Jul 2004 15:50:32 -0700

Can someone tell me why I'm having this problem:
When I select drink 5 and deposit too little money, there's a problem
with the 'difference' variable. Try to deposit $.80 instead of the
full $.85 and see what happens if you then keep entering .01 until you
have added the full .85. The program still asks the user to deposit
$0.00. It is telling me for some reason that 0.850000 - 0.850000 =
5.96046e-008.
Any suggestions?

/*
machine.txt:
------------------
Cola
0.75 20
Ruby Red Blast
1.00 10
Lemon Fizz
0.75 8
Grape Soda
0.90 5
Citrus Flip
0.85 0
Habanero Surprise
0.80 11
-------------------
*/

/* ================================================================ */
#include <stdio.h>
#include <string.h>
#include <math.h>
/* ================================================================ */
#define MAXNUMDRINKS 6
#define STRINGSIZE 25
#define noGood 0
/* ================================================================ */

typedef char String [STRINGSIZE];
typedef enum bool { false, true } bool;

typedef struct drinkRec
{
        String brand;
        float price;
        int quantity;
} drinkRec;

typedef drinkRec drinkList [ MAXNUMDRINKS ];

/* ================================================================ */

void ReadAllDrinks ( int* numDrinks, drinkList drinks );
void PrintDrinkMenu (int* menuChoice, int numDrinks, drinkList
drinks);
void GetNewQuantity (int menuChoice, drinkList drinks);
void GetMoney (int menuChoice, drinkList drinks);

/* ================================================================ */

int main ( )
{
        drinkList drinks;
        int numDrinks, menuChoice;
        double profit = 0;
        printf ("Welcome to the Soda Machine.\n");
        ReadAllDrinks ( &numDrinks, drinks );
        do
        {
                PrintDrinkMenu (&menuChoice, numDrinks, drinks);
                if (menuChoice != 0){
                        GetMoney (menuChoice, drinks);
                        if (drinks[(menuChoice - 1)].quantity > 0)
                                profit = profit + .25;
                        GetNewQuantity (menuChoice, drinks);
                }
        }
        while (menuChoice != 0);
        printf("\n%s%.2f\n","Current Run's Profit: $", profit);
        printf("\nBYE BYE\n\n");
        return ( 0 );
        
}
/* ================================================================ */

void ReadAllDrinks(int *numDrinks, drinkList drinks)
{
        String dummy;
        float dummy2 = 0;
        char *line;
        FILE *inFile;
        int count = 0;
        if ((inFile = fopen("machine.txt", "r"))) {
                while ((line = fgets(drinks[count].brand, STRINGSIZE, inFile))) {
                        drinks[count].brand[strlen(drinks[count].brand) - 1] = '\0';
                        fscanf(inFile, "%f%d",
&drinks[count].price,&drinks[count].quantity);
                        fgets(dummy, STRINGSIZE, inFile);
                        count++;
                }
                *numDrinks = count;
                fclose(inFile);
        }
        else printf( "machine.txt not read\n");
}
/* ================================================================ */
void PrintDrinkMenu (int* menuChoice, int numDrinks, drinkList drinks)
{
        int count, result;
        String dummy;
        printf("\n Drink Price Amount Left");
        printf("\n ------------- ----- -----------\n");
        for (count = 0; count < numDrinks; count++){
                printf("%d. %-19s$%4.2f", (count + 1),
drinks[count].brand,drinks[count].price);
                if (drinks[count].quantity < 1)
                        printf(" %8s","SOLD OUT\n");
                else
                        printf(" %8d\n",drinks[count].quantity);
        }
        printf("\n0. Exit\n");
        do
        {
                printf("\nPlease enter your selection: ");
                result = scanf("%d", menuChoice);
                if (result == noGood) /* This error checks for any input other
than an integer*/
                        scanf("%s", &dummy); /* If input is non-integer, force scanf to
stop looping */
        }
        while ((*menuChoice > numDrinks) || (*menuChoice < 0)|| (result ==
noGood));
}
/* ================================================================ */
void GetNewQuantity (int menuChoice, drinkList drinks)
{
        int prevQuantity;
        prevQuantity = drinks[(menuChoice - 1)].quantity;
        drinks[(menuChoice - 1)].quantity = prevQuantity - 1;
}
/* ================================================================ */
void GetMoney (int menuChoice, drinkList drinks)
{
        int result;
        String dummy;
        float moreMoneyIn, moneyIn, difference;
        if (drinks[menuChoice - 1].quantity > 0){
                do
                {
                        printf("\n%s%.2f: $","Please deposit $",drinks[menuChoice -
1].price );
                        result = scanf("%f",&moneyIn);
                        if (result == noGood) /* This error checks for any input other
than an float*/
                                scanf("%s", &dummy); /* If input is non-float, force scanf to
stop looping */
                }
                while (result == noGood);
                printf("\n%s$%.2f\n","You put in ",moneyIn);
                difference = (drinks[menuChoice - 1].price) - moneyIn;
                while (difference != 0){
                        if (difference > 0){
                                do
                                {
                                        printf("%s%.2f: ","Please deposit an additional $",
fabs(difference));
                                        result = scanf("%f", &moreMoneyIn);
                                        if (result == noGood) /* This error checks for any input
other than an float*/
                                                scanf("%s", &dummy); /* If input is non-float, force scanf
to stop looping */
                                }
                                while (result == noGood);
                                printf("\n%s$%.2f.\n","You put in ",moreMoneyIn);
                                moneyIn = moneyIn + moreMoneyIn;
                                difference = ((drinks[menuChoice - 1].price) - moneyIn);
                        }
                        if (difference < 0){
                                printf("%s%.2f","Your change is $", fabs(difference));
                                difference = 0;
                        }
                }
                printf("\n\nThank you, money accepted. Enjoy your drink!\n");
        }
        else printf("\n%s %s\n",drinks[menuChoice - 1].brand, "is sold out.
Please choose another.");
}